Photoproduction de pions neutres sur le proton au voisinage du seuil PDF Download

Author: Eddy Mazzucato
Publisher:
ISBN:
Category :
Languages : en
Pages : 220

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Book Description
A precise determination of the neutral pion photoproduction amplitude on the proton at threshold has been obtained from differential ross-section measurements in the energy region close to threshold (145-173 MeV). The experiment was carried out at the Saclay Linac by using a monochromatic photon beam obtained by in-flight positron annihilation. The photon tagging system allowed a precise energy determination of the incoming beam with a resolution of about 1 MeV. The signature of a good event consisted in the observation of the neutral pion decay gammas in lead glass Cerenkov counters, in coincidence with the re coil proton detected in an active target made of plastic scintillators. The good energy resolution achieved on the energy of the recoil protons provided a precise determination of the differentia cross-section. The analysis of the data in terms of multipoles gives a value for the threshold amplitude which is much smaller than the existing value extracted from higher energy experiments performed with untagged photon beams. The total cross section in the high energy region is, however, in fairly good agreement with old measurements. The experimental value of the electric dipole agrees with several calculations based on dispersion relations. Yet, our measurement cannot be reproduced by the low-energy theorems extrapolated to the physical pion mass. This significant discrepancy exhibits clearly the importance of dispersive contributions in the case of neutral pion photoproduction at threshold.